Solenoids are traditionally used to actuate mechanisms by the application of a voltage to an electromagnetic coil. Solenoids are expensive and require considerable design effort to ensure that the mechanical load requirements are consistent with the available force profile of the solenoid. This can be particularly challenging since the solenoid provides less force near the beginning of its stroke and provides exponentially more force as the stroke reaches the end of its travel. Solenoids suffer reliability problems because magnetic flux must bridge the plunger's sliding bearing and a residual magnetic force of close tolerance must prevent the plunger from magnetically sticking to the pole face. If either of these design parameters becomes too marginal, the solenoid performance is radically altered.
Linear actuators have been designed where a motor drives a threaded shaft and a corresponding threadedly coupled nut. The nut translates laterally when prevented from rotating by a guiding surface. The motor may be driven in one direction to emulate the drive stroke of a solenoid and driven in the other direction to return the nut and an attached carriage to a home position. To define the stroke of the motor driven linear actuator, axial stops have been used.
Prior actuators have used rod-shaped guides, the carriage engaging the guides to follow therealong. To ensure unrestricted movement, roller bearings typically engage the guides. The carriage can include an internal passage through which the threaded shaft passes. Bushings are typically coupled to the carriage at the ends of the internal passage to engage the threaded shaft in order to reduce wobbling.
However, a recurring problem with using bushings is that the bushings, which contact the rotating threaded shaft, tend to wear and create particles. The threads of the screw essentially act as a saw on the bushings. The resultant particles have been found to interfere with operation of the actuator. Particularly, the particles tend to gum up the actuator, causing it to lose response time and even completely stop functioning in some systems.
Another problem that has yet to be solved is how to reliably injection mold a portion or all of the carriage around a bushing. Standard materials often used in bushings cannot consistently withstand the high heat and pressures exerted on bushings during an injection molding process, and also withstand the rigors of everyday use. Particularly, traditional materials such as plastic-based or polymer-based bearing materials tend to deform in the hot injection mold. Oil impregnated bearings outgas during injection molding process due to the heat.
There is, therefore, a need for a linear actuator device that does not exhibit the wear problems inherent in the prior art.
There is also a need for a new bushing that is capable of reliably withstanding an injection molding process.